Update POST /messages HTML input to include

Update POST /messages HTML input to include all available HTML tags Webex outputs when sending a MarkDown

There is a lot of inconsistency in using HTML as an input in message creation. Technically Webex only supports h1,h2,h3,ul,ol,u,i,b and links. However, when using Markdown and retrieving the HTML field, you'll see Webex converted additional tags of li,br,strong,em,p,code.

These need to be support to better allow us to consistently format messages properly. In addition, would love to see actual <blockquote class='{{class}}'> support as this is instrumental in building messages automatically and calling out specific status elements.